Filamentous blue-green algae (cyanobacteria) provide an opportunity to elucidate the molecular mechanisms governing the formation of multicellular patterns. Just such patterns are formed, by little-understood processes called inductions and field effects, during ontogeny and regeneration of vertebrates and other higher organisms. Many neoplasias may entail loss of the multicellular coordination which underlies pattern formation. We have shown that intercellular interactions control the differentiation of vegetative cells into spores and heterocysts, thick-walled specialized cells, in the algal filaments. So as to identify the interactions involved, we are studying intercellular chemical interactions; we are bioassaying for developmentally active substances; and we here propose to isolate conditional mutants in which, as one of several benefits, differentiation will be greatly increased as a consequence of modified intercellular interactions. We expect that analysis of pattern formation in these morphologically simple organisms will facilitate analysis of similar phenomena during normal development of vertebrates. It might then become possible, by imposing coordination, to facilitate organ regeneration, and to stabilize or cause reversion of certain kinds of neoplastic growth.